The present invention relates to a strain sensor with an optical fiber, which has a fiber Bragg lattice, in the following referred to as FBG strain sensor. The invention is suited to accurately measure a strain also on curved objects.
Strain sensitive sensors, which are fastened on material surfaces to be investigated are known as so-called metal foil DMS or FBG fiber sensors. A metal DMS essentially consists of a plastic carrier foil on which a thin meander-shaped metal foil strip is fastened over its entire surface. For detecting strain, the plastic carrier foil is glued onto the material surface to be investigated. The use of a plastic foil as sensor carrier is necessary because only such an arrangement allows application of DMS in a defined manner. An electrically insulated application of the very thin metal foil without the relatively stable plastic foil is not practicable outside laboratory conditions.
In analogy to these metal foil DMS, FBG sensors were developed, which as essential component have a glass fiber with a fiber Bragg lattice, in the following referred to as FBG. This glass fiber also has to be fastened on the material surface to be investigated. The fact that the glass fiber is thin and fragile also poses problems during handling. Therefore, a sensor carrier had to be developed in order to make the FBG sensor less sensitive to rough praxis conditions. Such sensors are described in the documents JP 2003 279760 A and WO 2008/101657 A1. Embedding the sensitive fiber with the FBG into a soft plastic compound enables handling of the sensor and with this makes it suitable for use in praxis. The FBG sensor described in the document WO 20081101657 A1 has a two-point force introduction which results in a significantly greater measuring accuracy compared to FBG sensors which are applied over their entire surface. With this type of sensor, strains can be precisely measured on even surfaces. In contrast to metal foil DMS however, problems arise during measurements with FBG sensors on curved surfaces, which are explained by way of FIG. 1a to 1g. 
FIGS. 1a-1c show a conventional metal foil DMS, which is glued onto a curved material surface, wherein FIG. 1a shows a perspective view of the curved material surface with a DMS application. FIG. 1b shows the side view of FIG. 1a and FIG. 1c shows an enlarged section of FIG. 2b. 
When the material is strained as a result of the influence of force or temperature as shown in FIG. 1c by the double arrow A1, the fastening of the carrier foil over the entire surface by gluing onto the material causes the strain to be transmitted evenly to the metal foil which is connected over its entire surface with the carrier foil and with this the metal foil is also strained to the corresponding degree. Insofar the material strain is transmitted to the metal foil DMS almost error-free as indicated with the double arrow A2 in FIG. 1c. 
However, an FBG sensor with a two-point force introduction involves a different type of strain transmission, which is explained in the following. FIGS. 1d and 1e show a longitudinal sectional view and a cross sectional view of an FBG sensor fastened on a material according to the document WO 2008/101657 A1, which is constructed as follows: A glass fiber with an FBG is held between two rigid fastening elements and is embedded in a soft plastic, for example silicone rubber. The two rigid fastening elements are glued onto the material surface to be investigated. The soft plastic preferably serves for protecting the section of the glass fiber, which section is provided with the FBG, from interfering forces, i.e., lateral forces, and for improving the overall handling of the FBG sensor during application. However, when an FBG sensor with this construction is used on a curved surface, effects occur which lead to measuring errors. The causes for this are explained in the following by way of FIGS. 1f-1g. 
FIG. 1f shows the sensor applied onto a curved material surface according to the document WO 2008/101657 A1, and FIG. 1f shows the manner in which the glass fiber follows a strain of the material surface. The strain of the material surface is indicated by the double arrow in the material to be monitored. As a result of the strain of the material surface the distance between the two fastening elements is increased by the lengths a+a, i.e., the fiber is strained by the length 2a. Because the fiber is only clamped in the two fastening elements it is pulled in the direction of the material surface, i.e., the plastic presses indiscriminately onto the FBG. In addition this movement of the fiber results in falsified transmission of the actual strain of the material surface onto the FBG, so that a measuring error occurs which depends on the radius of curvature of the material surface.
Therefore it was found that an accurate strain measurement on curved surfaces is neither possible with a sensor according to JP 2003 279760 A nor with the sensor WO 2008/101657 A1.